WASHINGTON -- After a rocky July interrupted his stellar rookie season, Nationals reliever Aaron Barrett was optioned to Triple-A Syracuse before Friday night's game against the Phillies to open a roster spot for newcomer Asdrubal Cabrera.

Fellow rookie right-hander Blake Treinen, recalled Thursday, will remain in the bullpen to provide an extra long man while Barrett tries to straighten some things out in the Minors.

"Aaron has been in a lot of pressure situations so far, and it's not easy for a first-year pitcher to be in those situations," Nats manager Matt Williams said. "He's pitched really well. But this gives us an opportunity to get him down and work on things and gave him more of a structured environment to pitch in."

In his first 30 appearances through June 27, Barrett sported a 1.67 ERA over 27 innings, allowing 20 hits and 14 walks while striking out 33. Batters hit .213 against him, with a .571 OPS.

Barrett's season seemed to turn in an appearance against the Rockies on June 30, when home-plate umpire Joe West called him for a balk stemming from the same pre-pitch routine he had used since 2011. Forced to alter his delivery, Barrett has allowed 10 runs (seven earned) on nine hits and four walks in his last 6 2/3 innings over 10 games, giving up a .281 average and .830 OPS.

"I don't think it helps," Williams said of the changes Barrett had to make. "We're all creatures of habit and we're all creatures of timing in this game, so if you have your comfort level up and your timing's right, things tend to work out. When something like that is put into the mix, then it disrupts that."

With Barrett gone and Treinen available to eat innings after starting at Syracuse, Williams will have more freedom to deploy righty Craig Stammen and lefty Ross Detwiler earlier in games rather than saving them for potential extra-innings situations.

Meanwhile, Barrett can work on getting settled in his delivery and stay fresh for the stretch run, as he sits only 15 2/3 innings shy of his career high.

"You never want to lose him," Williams said. "But you try to pick the right opportunity to do so, and we felt this was the right opportunity."